1. Field of the Invention
The present invention relates to a method for the treatment of Electric Arc Furnace (EAF) dust so as to especially extract zinc.
2. Discussion of Related Art
A commonly used in the recycling of scrap metal containing zinc is the electric arc furnace (EAF) in which the scrap metal is melted by forming electric arcs between the graphite electrodes and the scrap metal. During the recycling process, zinc and other metals are evaporated as dust forming minute particles of the oxides. The dust typically contains 10-40% of zinc, 20-40% of iron, about 5% of lead, and others such as chrome, cadmium, copper, tin, manganese, quartz sand, alumina, limestone, sulfides, chlorides, etc.
Motivated by strict environmental regulations, the dust can be no longer simply disposed of as landfill because of the possible danger of contamination of soil and rivers by the hazardous metal components such as lead, copper, cadmium, etc., being leached out from the dust. Thus, it is important to treat the dust containing noxious heavy metals in a safe and economical manner, and many methods have been suggested to economize the expenses of the electric arc furnace dust disposal by extracting and recycling zinc from the electric arc furnace dust.
Pyrometallurgical and hydrometallurgical techniques are well known methods for extracting zinc from the electric arc furnace dust.
At the cost of high extraction yield of zinc from the dust, the pyrometallurgical processes require reduction agent of high costxe2x80x94such as cokesxe2x80x94and high temperature ambience. The pyrometallurgical processes further require expensive facilities for separating the zinc from the dust and produce crude zinc oxide with low commercial value.
The hydrometallurgical processes can produce high-purity metallic zinc or zinc oxide in comparison with the pyrometallurgical method but most of them cannot completely leach zinc ferrite contained in the dust as a major phase.
Hot acid leaching was found useful to leach zinc ferrite particles in EAF dusts. There has been growing trends to utilize commercially the processes for extracting sulfuric acid and zinc from ZnS, and many methods have been suggested in regard to leaching and extracting zinc from EAF dusts with sulfuric acid.
However, most processes using sulfuric acid require high temperature and high-pressure conditions in order to avoid jarosite formation. Another disadvantage is leaving harmful elements, lead, cadmium and copper, unleashed. Furthermore, in the zinc electrodeposition processes from a sulfuric solution, EAF dusts normally contain a few percent of the noxious chloride which should be removed for the following zinc extraction stage due to aggressive behavior to anode materials in sulphate system.
Compared with sulfuric acid, hydrochloric acid was found as a very effective lixivant to leach zinc ferrite from EAF dusts with high zinc yield and without jarosite formation.
Hydrochloric acid has chemical activity 4-10 times as high as sulfuric acid of the same concentration and is widely used as a very appropriate leaching reagent because its activity increases with the increase in the concentration. The hydrochloric acid advantageously generates no jarosite that is difficult to eliminate.
That is, the processes using hydrochloric acid can leach more than 90% of zinc within short time at low temperatures with high leaching efficiency and can leach almost all heavy metals. The processes also allow the size of a reactor to be smaller due to high saturation degree of zinc and to extract iron in the form of ferric oxide. Furthermore, the processes using hydrochloric acid can be used irrespective of metal salts existing in EAF dusts.
Accordingly, requirements for collecting zinc from EAF dusts with hydrochloric acid are as follows: (1) the hydrochloric acid reagent should be recycled economically with high yield for the purpose of practical use of the combination of the processes comprising leaching and electrolytic extraction; (2) the product thus obtained should be valuable with the least harmful residue to compensate the disposal expenses ; and, (3) the system should be safe and simple but not occupy much space.
U.S. Pat. No. 4,572,771 discloses a method for leaching the EAF dusts in a 75-200 g/l HCl solution associated with an electrolytic extraction cell of a sulfuric acid system using an organic solvent extraction technology.
U.S. Pat. No. 4,610,722 discloses a method for leaching the EAF dusts with a mixed solution of sulfuric acid and hydrochloric acid and the subsequent solvent extraction technology.
U.S. Pat. No. 5,336,297 involves a method for leaching the dusts with a ferric chloride solution under high temperature and high pressure conditions to extract hematite and then using a solvent extraction technology.
U.S. Pat. No. 5,709,730 involves a method for leaching the dusts with a mixed solution of calcium chloride and hydrochloric acid and then collecting hydrochloric acid with sulfuric acid to produce gypsum.
The above-mentioned patents employ the solvent extraction technologies. However, trial experiments performed to examine the economical or technical feasibility of the patented methods revealed that the patents were disadvantageous in that the system concerned occupies much space and requires a large installation expense.
Accordingly, an object of the present invention is to provide a method for extracting high purity hydrochloric acid and zinc from an impure zinc chloride solution and, specifically, a method for extracting hydrochloric acid and zinc from electric arc furnace dust containing zinc ferrite by purification using active carbon and zinc powder, directly extracting hydrochloric acid with a cation exchange membrane, and eliminating chlorine gas generated during electrodeposition with active carbon.
To achieve the first object of the present invention, there is provided a method for extracting hydrochloric acid and zinc from EAF dusts containing zinc ferrite including the steps of: (a) hot acid leaching of solids remaining in a neutral leaching step using a mixed solution of hydrochloric acid collected from an electrolytic extraction cell (electrowinning cell) and zinc chloride, to prepare a zinc chloride solution; and (b) neutral leaching by adding new dust to eliminate iron dissolved in the hot acid leaching and introducing air and oxygen generated from the electrolytic cell.
According to the present invention, there are included a first purification step using zinc oxide for eliminating a small amount of iron existing after the neutral leaching, a second purification step using active carbon for eliminating organic matters dissolved in the dust, and a third purification for eliminating Pb, Cu and Cd through cementation.
The present invention includes an electrodeposition step for electrolytic extraction of zinc from the purified solution. The electrodeposition step is performed in a cell having a cation exchange membrane. The present invention further includes a chlorine gas eliminating step for passing the purified solution through a heating unit for removing the solution of chlorine gas generated from an anode of the electrolytic cell. From the electrolytic cell is discharged an electrolyte having acidity of 1-2N HCl suitable for hot acid leaching.
Furthermore, the present invention decomposes an organic metal complex deteriorating current efficiency and uses zinc powder for eliminating other heavy metals through cementation.
Zinc and hydrochloric acid are collected from the purified solution in an electrodeposition chamber using the cation exchange membrane. The current density for this process is 300-1000 A/m2 for the cation exchange membrane and 750-2000 A/m2 for the cathode. Usually, the membrane current density is higher than the cathode current density.
An electrolyte contains 50-130 g/l Zn and 0-40 g/l hydrochloric acid (except for zinc chloride) and the used electrolyte contains 104-270 g/l Zn and 37-74 g/l hydrochloric acid (except for zinc chloride).
The electrodeposition chamber has a structure with cathodes and anodes alternately arranged. The anode is of a tabular structure surrounded by the cation exchange membrane. A 1-2N sulfuric acid solution is introduced into the anode to cause generation of oxygen and about 2% of hydrochloric acid is lost during the electrodeposition. In order to eliminate chlorine generated during the electrodeposition, an anode solution discharged from the electrolytic cell is heated to above 90 degree Celsius to vaporize the chlorine and introduced into the electrolytic chamber again.
According to the above-described zinc extraction method, the present invention uses hydrochloric acid and a zinc chloride solution for leaching in order to reuse the electrolyte discharged from the electrolytic extraction cell as a lixivant. As well known, the leaching rate of zinc ferrite largely depends on the hydrogen activity of the solution and an increase in the zinc chloride concentration increases the hydrogen activity in the aqueous chloride solution. In the present invention, the acidity is maintained below 2N that is enough to leach 100% of zinc ferrite particles.
According to several experimental tests for zinc chloride electrolysis, the present invention has economical feasibility with regard to disposal of the dust in that one membrane cell is selected to reduce the costs of membrane and prevent additional power consumption in an anion exchange membrane.